It is apparent that in future both static applications, for example in the case of wind power installations, and vehicles, for example in hybrid and electric vehicles, will make increasing use of new battery systems on which very great demands in terms of reliability are made.
The background to these great demands is that failure of the battery system can result in failure of the entire system. By way of example, in an electric vehicle, power failure of the traction battery results in what is known as a “breakdown”. Furthermore, the failure of a battery can result in a safety-related problem. In wind power installations, for example, batteries are used in order to protect the installation against inadmissible operating states in a high wind by means of rotor blade adjustment.
The block diagram for a battery system based on the prior art is shown in FIG. 1. A battery system denoted as a whole by 100 comprises a multiplicity of battery cells 10 which are combined in a module 24. In addition, a charging and isolating device 12, which comprises an isolating switch 14, a charging switch 16 and a charging resistor 18, is provided. In addition, the battery system 100 may comprise an isolator device 20 having an isolator switch 22.
For safe operation of the battery system 100, it is absolutely necessary for each battery cell 10 to be operated within a permitted operating range (voltage range, temperature range, current limits). If a battery cell 10 is outside these limits, it needs to be removed from the cell complex. If the battery cells 10 are connected in series (as shown in FIG. 1), failure of an individual battery cell 10 therefore results in failure of the entire battery system 100.
Particularly in hybrid and electric vehicles, batteries in lithium ion or nickel metal hybrid technology are used which have a large number of electrochemical battery cells connected in series. A battery management unit is used to monitor the battery and is intended to ensure not only safety monitoring but also as long a life as possible. By way of example, a cell voltage sensing unit is thus used.
FIG. 2 shows the known use of a contemporary cell voltage sensing unit.
FIG. 2 shows an architecture which is known from the prior art for typical cell voltage sensing. In this case, each module 24 with its battery cells 10 has an associated cell voltage sensing unit 26. The cell voltage sensing unit 26 comprises a multiplexer 28 which senses the voltage of each of the individual battery cells 10 by using a number of channels 30 which corresponds to the number of battery cells 10. The multiplexer 28 is connected via an analog-to-digital converter 32 to a gateway 34 which is coupled to a communication bus 36. The communication bus 36 has a central microcontroller 38 connected to it. This central microcontroller 38 can therefore be used to sense and evaluate the voltages of the individual battery cells 10. The microcontroller 38 may be part of a battery management unit.
As clarified by FIG. 2, a plurality of modules 24 having battery cells 10 may be arranged in series in this case, said modules each having a dedicated cell voltage sensing unit 26.
The multiplexers 28 have auxiliary inputs 40, which are indicated here, which are known to be able to be used for temperature measurement by allowing resistance values of NTC resistors to be sensed.
A drawback of the known cell voltage monitoring is that a malfunction in the cell voltage sensing unit, particularly in the analog-to-digital converter, cannot be identified. The data transmitted by the analog-to-digital converter are considered by the evaluation unit to be the actual voltage values as provided. If these are erroneous, however, the entire battery system may behave incorrectly.